In an interview on Fox News, where he had been a contributor, shortly after the news broke, Bolton called his appointment a "great honor" but seemed surprised that the Trump administration had announced the appointment so soon. Bolton was spotted earlier Thursday at the White House.

"It's always an honor to serve our country, and I think, particularly, in these times, internationally, it's a particular honor," Bolton said.

But when pressed by host Martha MacCallum about his foreign policy views on a variety of pressing topics, from the Iran nuclear deal to North Korea, Bolton repeatedly deflected.

Bolton did say he was "outraged" by a leak earlier this week that apparently came from someone at the National Security Council. According to the Washington Post, despite an all-caps warning in Trump's security briefings not to congratulate Russian President Vladimir Putin on his electoral victory last weekend, the president did so anyway.

Bolton also argued that Trump's decision to congratulate Putin "wasn't a significant point one way or the other."

"I've said 'congratulations' to a lot of people — foreign diplomats and officials. It's a matter of being polite," Bolton said.

Trump has accepted an invitation to meet North Korean leader Kim Jong Un as early as May. He also has to weigh in on the Iran nuclear deal again that month. Bolton has been an advocate for regime change in both countries and, as a senior fellow at the conservative American Enterprise Institute, he wrote that President Trump should "abrogate the Iran nuclear deal in his first days in office."

Instead, Trump has kept up the U.S. side of the bargain, continuing sanctions relief in exchange for limits on Iran's nuclear program. He is warning he could walk away from it in May unless diplomats fix parts of the deal he doesn't like by then.

"No fix will remedy the diplomatic Waterloo Mr. Obama negotiated," Bolton wrote in the Wall Street Journal in January. "Mr. Trump correctly sees Mr. Obama's deal as a massive strategic blunder, but his advisers have inexplicably persuaded him not to withdraw," he added.

When it comes to his hawkish views on Iran, Bolton is more in line with the newly nominated secretary of state, Mike Pompeo, who still has to be confirmed for the job. The national security adviser doesn't need to go through that confirmation process, which may have been difficult for Bolton. He is the third man to hold the job for the Trump administration, following McMaster and before that Michael Flynn.

Also on Fox News Thursday, Bolton described the job he was about to undertake as having two primary facets. The first, he said, was that of being an "honest broker" to the president — "making sure that the president has the full range of options presented to him to make the decision that only the president can make," which is "presented in a way that gives the president a chance to weigh the pluses and minuses."

The second part of that job, Bolton said, was to be a conduit between the White House and other agencies and offices tasked with implementing the president's decisions, "making sure that the bureaucracies out there get the decision and implement it."

Bolton said that hasn't always been a smooth process, but that "the rest of the bureaucracy needs to understand as well that when the guy that got elected makes a decision, that's what the Constitution provides."

The new national security adviser-designate also said he believed there should be a "free interchange of ideas among the president's advisers," otherwise "the president is not well-served."

Bolton served as undersecretary of state for arms control and United Nations ambassador during the George W. Bush administration and was an advocate for the Iraq War. He stepped down as ambassador in 2006 after holding the job on a temporary basis and facing a challenging path to confirmation in the Senate.

He advocates a sort of "drain the swamp" agenda when it comes to international organizations, calling, for instance, to move from "assessed funding" to "voluntary funding" at the United Nations. He once famously said that if the U.N. Secretariat building lost 10 stories, "it wouldn't make a bit of difference."

Bolton rose to prominence as a lawyer for the Bush administration during the Florida recount and was well-known at the State Department's headquarters at Foggy Bottom for his lawyerly arguments and hard-line approach.

Democrats were quick to point out Bolton's controversial past. House intelligence committee Ranking Member Adam Schiff, D-Calif., suggested in a tweet that McMaster's departure and Bolton's appointment would put "our nation's security at risk."